This is my first post here so first let me say that I think REW is a great program and I really appreciate the authors making this available for free.
I think I have seen what Hannes is talking about as shown below
squished REW graph.jpg
I get this by using the minus (-) zoom when viewing a plot. Zooming back in or using the graph limits button usually fixes it. In the above graph I set the limits to 100-200 and started clicked the (-)zoom. It is more pervasive if you use wide X axis like 100-1000. I think I've seen it under other conditions but I can't recall the exact circumstances.